1. Field of the Invention
This invention relates generally to the field of magnetron sputtering and more specifically to linear magnetron apparatus for enhancing the uniformity of a plasma sheath along an elongated target to provide a uniform sputtering rate along the length of the target and to provide a uniform thin film coating on the substrate.
2. Brief Description of the Prior Art
Sputtering is a well-known process in which the surface of an object or substrate is coated with a thin film of material that is sputtered, i.e., physically eroded by ion bombardment, from a target. Sputtering is implemented by creating an electrical plasma over the surface of the target material in a low-pressure gas atmosphere. Gas ions from the plasma are accelerated by electric fields to bombard and thereby eject atoms from the surface of the target material. These atoms travel through the gas environment until they impact the surface of the object or substrate where they bond, creating a coating layer. Sputtering has most commonly been used in the semiconductor industry where thin metal coatings are formed on semiconductor substrates and then later patterned to form the various conductive contacts, interconnections, and insulating surfaces on such substrates. However, an ever-expanding variety of products are being coated by this sputtering process, including, for example, architectural glass, computer screens, sheet steel, sunglasses, automobile parts, automobile glazing, surgical implants, jewelry, tool bits, sheet plastic, fabrics, and fiber optics. Generally speaking, the articles to be coated have been of a flat or planar nature.
In a typical DC magnetron sputtering operation, a target of the material one wishes to use for a coating is placed within a low pressure gas plasma and connected as a cathode. Ions from the gas, most usually a chemically inert noble gas such as argon, bombard the surface of the target and knock off atoms of the target material. The object being coated is typically placed with respect to the cathode such that it is in the path of the sputtered atoms. Accordingly, a thin film of the material is deposited on the surface of the object. It is the nature of the sputtering process that the sputtered atoms leave the target surface with relatively high energies and velocities such that when the atoms bombard the substrate surface they actually intermix into the atomic lattice of the substrate surface, creating a penetrating tight bond.
While the overall yield of the sputtering process, that is, the number of atoms sputtered per incident ion, depends on the energy of the incident ions, the overall sputtering rate not only depends on the energies of the incident ions, but also depends on the number of ions impacting the target surface. The ion energy and number of ions are dependent on the level of ionization in the gas plasma (glow discharge) and also upon the location of this plasma with respect to the target surface. Therefore, it is desirable that the ions in the plasma be produced adjacent the target surface, so that their energies are not dissipated by collisions with intervening gas atoms.
A standard method of improving overall efficiency of glow discharge sputtering has been to use magnetic fields to confine electrons to the glow region in the vicinity of the cathode target surface. The addition of such magnetic fields increases the rate of ionization. Numerous types of magnetron sputtering apparatus, as they have come to be known, have been developed for this very purpose. Essentially, electrons emitted from the target surface will accelerate to a drift velocity that is orthogonal to both the directions of the electric field and the magnetic field as measured over the surface of the target. In almost every magnetron sputtering device, the paths travelled by these electrons close on themselves forming a closed loop. Further, the magnetic fields of such devices are typically designed such that arching lines of flux form "tunnels" through which the electrons travel at drift velocity. That is, the electrons travel generally in a "ring" or "racetrack" configuration: As they rise from the target surface, the electrons whirl around the racetrack loop in proximity to the target surface, thereby increasing sputtering rate, as discussed in the patents issued to Chapin, U.S. Pat. No. 4,166,018; Class, U.S. Pat. No. 4,198,283; McLeod, U.S. Pat. No. 3,956,093; and Corbani, U.S. Pat. No. 3,878,085.
In magnetron type sputtering devices, the efficiency of plasma generation is increased because all of the electrons caught in the magnetic field have an increased effective path length in the proximity of the target. That is, each electron emitted from the target surface has a much longer distance of travel while in the proximity of the target. The result is that the electrons have collisions with much higher numbers of gas atoms while near the target. Accordingly, the resultant higher intensity plasma has more ions available to bombard the surface resulting in a higher sputtering rate. Another method of increasing the sputtering rate is described by Cuomo in U.S. Pat. No. 4,588,490. Essentially, Cuomo adds an additional electrode, known as a hollow-arc cathode as a third and separately biased electrode to increase the numbers and energy of electrons generally available in the vicinity of the target. However, a disadvantage associated with the addition of such a hollow cathode is that the hollow cathode must be made of refractory metal because it operates at extremely high temperatures. Another disadvantage associated with Cuomo's hollow cathode is that it must be operated with a gas flow through the arc cathode that needs to be appropriately controlled for proper operation. Yet another disadvantage is the need for an additional high-current biasing supply that must be set at an appropriate biasing level. In general, Cuomo does achieve higher rates because he is working in the right direction; higher rates are achieved by a "hotter" plasma most proximate to the cathode surface. However, this increased sputtering rate comes at the expense of a relatively large increase in the number of additional components and complex control apparatus.
Magnetron sputtering systems can be classified into various groups. Principal among these groups are the circular, planar, and cylindrical types of sputtering apparatus. Circular and planar magnetron apparatus tend to suffer from non-uniform erosion which renders the target unusable even when relatively large amounts of useful target material still remain. This non-uniformity of erosion is a direct result of the "racetrack" effect described above. That is, the plasma is more intense over the racetrack area and sputters the target material directly below the racetrack at a vastly greater rate than the portions not directly below the racetrack. After this portion of the target material is eroded away, the target must be replaced even though material remains at the center and at the edges of the racetrack so that the device will not sustain damage to the supporting cooling structure. Since many sputtering applications require expensive high purity target materials, this is an uneconomic and wasteful circumstance. Actually, any of the current sputtering devices having a closed flux-tunnel electron loop or racetrack configuration will suffer non-uniform target erosion, since the plasma concentration necessarily varies over the surface of the target. Another disadvantage of circular and planar magnetrons is that they tend to have quite bulky magnet structures making them large and cumbersome. Because of this bulky structure, such magnetrons are generally useless for creating uniform films inside concave or cylindrical structures.
Several different types of cylindrical magnetron sputtering devices have been developed in part to solve the non-uniformity problems associated with the circular and planar devices, as described above. Basically, the target material in a cylindrical device is in the form of an elongated tube. Confining magnetic fields are usually provided by complex bulky solenoid coils disposed within the target around the outside of the magnetron chamber as described in the patents issued to Penfold, U.S. Pat. Nos. 3,884,793 and 4,031,424. Penfold uses solenoidal coils to generate magnetic fields having flux lines parallel to the axis of the elongated cathode target. Such cylindrical magnetrons tend to be somewhat more even in their erosion patterns, however, they suffer from undesirable end effects. That is, in a cylindrical magnetron the direction of the electron drift velocity vector causes the electrons to orbit around a central target post. Unfortunately, however, the electrons tend to leak out or escape their orbits near each end of the central post, resulting in lower ionization intensities, thus lower sputtering rates, at each end of a cylindrical target. Further, such cylindrical magnetrons are also expensive to construct and are not suitable for coating substrates with large planar surface areas. Another disadvantage associated with the cylindrical magnetrons disclosed by Penfold is that the elongated target may not be bent or shaped to follow the contours of irregularly shaped objects. If the target in a cylindrical magnetron, such as that described by Penfold, is bent or shaped to follow the contours of an irregularly shaped object, the magnetic field strength over the target surface would not be uniform, resulting in marked non-uniformities in the plasma sheath, thus, the sputtering rate, along and around the surface of the target. Further, because the magnetic field is axially oriented and parallels the longitudinal axis of the elongated cathode of the target assembly, it is difficult, if not impossible, to reshape the magnetic field so that it maintains the necessary uniform field strength over the curved or bent target surface. Accordingly, such cylindrical sputtering devices utilizing axially oriented magnetic fields are not useful in uniformly coating irregularly shaped objects. McKelvey, U.S. Pat. No. 4,445,997, realizes a fix for a somewhat non-planar surface, but his solution to the problem is a device with multitudes of parts and a target surface that would be very expensive to form with high-purity materials. Finally, a further disadvantage of these types of cylindrical sputtering devices is that they are not useful for coating the inner surfaces of small diameter magnetic steel tubing, because the magnetic field produced by the solenoid outside of the tube cannot penetrate into the internal regions of the tube.
The patent issued to Zega, U.S. Pat. No. 4,376,025, attempts to solve some of the problems associated with the above-described cylindrical magnetron sputtering apparatus by reorienting the magnetic flux lines circularly around the axis of the elongated rod-like target material, as opposed to the axial orientation used by Penfold. Essentially, Zega discloses a cylindrical magnetron device utilizing a tubular current-carrying electrode disposed within a tubular target cathode. Instead of using a separate solenoid coil to generate the plasma-confining magnetic field, Zega uses a high-current carrying hollow electrical conductor disposed within a tubular target to generate a circumferential magnetic field that surrounds the tubular target. Zega presents data that show the device achieves very poor uniformity of coatings. There is some improvement gained by the use of AC driving potential for magnetic field generation, however, even depositions of coating are still not possible. A further disadvantage of Zega's device is that his cylindrical magnetron is still not useful for coating large planar surfaces. Unlike the prior art magnetrons, the Zega magnetron, despite its poor sputtering uniformity, does not depend on a closed-loop flux tunnel.
Corbani, U.S. Pat. No. 3,878,085, did try to make a similar embodiment work for coating of planar surfaces. One of his patent embodiments shows a magnetron structure that is open-ended with no looped magnetic field. He complains, however, that
"the embodiment of FIG. 6 has the disadvantage that its path has a beginning and an end, and that charged particles will be ejected from the end. Also, since there is a beginning, sputtering will not occur for an initial length of the path, and part of the device is not functional for generation of sputtered material."
The disadvantage of Corbani's device is that it produces sputtering rates of different magnitudes longitudinally along the surface. In operation, then, Corbani's device would give a rate curve similar to that shown in FIG. 2a of Zega's U.S. Pat. No. 4,376,025.
In summary, none of these prior art DC magnetron sputtering devices are capable of achieving a uniform sputtering rate over the entire surface of an irregularly shaped target without going to great pains and expense, such as in the device disclosed by McKelvey. There is a great need for an inexpensive device that could be simply fashioned to coat such surfaces. However, to do so would ideally require a target that is shaped to "silhouette" the object so that the object being coated can be brought adjacent the target in such a way that the distance between all points on the surface of the object and the target are substantially equal. Unfortunately, prior art sputtering devices had to go to extreme expense and complexity, sometimes sacrificing target use or power efficiency and almost always introducing a large number of parts with a great surface areas into the vacuum system, thus increasing the overall cost and complexity of the system. Moreover, large surface areas and great numbers of parts are antithetical to the creation of quality-high purity films in that they tend to poison the sputtered film with unwanted impurities.
Furthermore, the aforementioned prior art devices tend to have relatively large targets of much greater lengths or diameters than would otherwise be necessary if such devices could achieve a highly uniform plasma sheath over the target surface to provide at least an acceptable degree of uniformity of the film deposited onto the substrate.
Therefore, there remains a need for a sputtering apparatus that is capable of uniformly and efficiently depositing a thin film of the target material on objects having various shapes, such as the inner surface of crucibles, the inner diameters of cylinders, shaped automobile glazing, and radically curved lenses. Such an apparatus should achieve a uniform and intense plasma sheath evenly over the full surface of the target to achieve uniform erosion and high sputtering rates. Further, such an apparatus should have a target that can be easily placed in close proximity to the surface being coated to increase the deposition efficiency and alleviate the problems caused by target atoms being sputtered on objects in the vacuum system other than the substrate. Ideally, such a sputtering apparatus should allow a wide variety of shapes, sizes, length, and widths of target cathode surface to be used while maintaining uniform fluxes of sputtered atoms along the entire length of the target cathode surface. Such a sputtering device should also be efficient, have a reasonable freedom from generating contamination and particulates and still be relatively easy and inexpensive to manufacture. Prior to this invention, no such sputtering device existed.